1. Field of the Invention
This invention relates to the resolving of emulsions having a continuous oil phase containing a dispersed water phase. More particularly, the present invention relates to electrical field treatment for resolving water-in-oil emulsions.
2. Description of the Prior Art
Electrical field treaters have been employed to resolve emulsions for nearly 70 years. These emulsions have a continuous oil phase, which terminology includes the various hydrocarbons such as crude oil, petroleum distillates and residuum derived during refining and other hydrocarbon processing. Also included in this terminology are various organic liquid materials which are water immiscible, have a relative low electrical conductivity and a low dielectric constant compared to water. The dispersed water phase may be water, brine, or other aqueous material such as acids or caustics. When the continuous phase is crude oil, the treater usually employs an a.c. electric field for resolving the emulsion. A d.c. electric field is preferable for resolving the dispersion where the continuous oil phase is a distillate and the dispersed water phase is an acid or caustic. In other instances, it is desired to first apply an a.c. electric field to the emulsion, and then, subjecting the emulsion to a d.c. electric field to complete the treatment. Reference may be taken to U.S. Pat. No. 2,855,356 wherein there is described such a combination a.c.-d.c. electrical field treater for resolving an emulsion formed of distillate and an aqueous treating material. More particularly, the electrical field treater is arranged for vertical flow of the emulsion and employs two sets of electrodes. The upstream set of electrodes is energized by an a.c. potential and the downstream set of electrodes is energized by d.c. potential. In addition, these a.c.-d.c. electric fields are arranged to provide potential gradients which change from non-uniform to uniform character in the direction of emulsion flow. In this treater, each set of electrodes has one electrode energized and the other electrode referenced to ground for defining the electric field. A subsequent improvement is illustrated in U.S. Pat. No. 2,897,251 employing a dual output d.c. power source to provide two separate d.c. potentials for energizing separate insulated sets of electrodes, one electrode in each set being referenced to ground. A related combination of a.c.-d.c. electrical field treater is shown in U.S. Pat. No. 2,849,395. In this reference, a dual zone electrical field treater has an upstream a.c. electric field zone with a following downstream d.c. electric field zone for treating emulsions. In particular, the d.c. zone employs a novel arrangement of planar, vertically-oriented, metal electrode sets, all of which are insulated from the metal vessel and energized through separate entrance bushings from an external power source. The external d.c. power source has a full wave or half wave rectifier for producing the desired d.c. electric field. In this instance, both electrodes are energized relative to earth ground at a first potential gradient, and the sets of electrodes are energized at a second potential relative to one another.
In the foregoing references, it is noted that the a.c. electric field and the d.c. electric fields for the combined a.c.-d.c. electrical field treaters were provided separately by an a.c. power source and a d.c. power source. However, it has been proposed to employ a single power source with an electrical field treater to energize simultaneously sets of electrodes to a.c. and d.c. electric fields for emulsion resolution. In U.S. Pat. No. 2,849,395, there is shown an electrical field treater for resolving an emulsion into a treated oil phase and a coalesced water phase using a.c.-d.c. electric fields. Parallel, vertical metal plates are arranged within a metal vessel with one set of electrodes positioned upstream for energization by an a.c. voltage and a downstream set of electrodes energized by a d.c. voltage, which preferably is pulsed. The metal vessel forms a subtended grounded electrode beneath these electrode sets. As a result, the grounded electrode (and any water layer) of the vessel serves as a circuit common to both a.c. and d.c. voltages. The power source provides simultaneously both d.c. and a.c. potentials on the electrodes relative to earth ground. The potential gradient (a.c. and d.c.) between electrodes in the upstream and downstream sets is uniform between adjacent electrodes in each set and also relative to the metal vessel which forms the circuit common reference.
Attempts have been made to provide a common a.c.-d.c. power source for energizing separate insulated electrode sets with an a.c. voltage component on electrodes upstream of the electrodes energized with a d.c. voltage. For example, U.S. Pat. Nos. 3,772,180 and 3,847,775 illustrate an electrical field treater with a power source described as providing combined a.c.-d.c. potentials for energizing an insulated set of electrodes. The power source has a high voltage secondary winding with one terminal at earth ground and the other terminal common to both electrode sets through half wave rectifiers. If the current flow to the electrodes is of similar magnitudes, the system theoretically would be operable. However, a slight unbalance in the current consumed by either electrode causes current within the secondary winding of the transformer. Obviously, a d.c. current in the secondary winding of the transformer causes magetization of the transformer core leading to increase in the primary current. This in turn will lead to saturation of the current regulator preceding the transformer, and the primary overcurrent protection means will most likely disconnect the power source from the main supply. Thus, unbalance between the current passed by the rectifiers to their respective loads will result in the necessity of disconnecting the source from the main supply or require that components for regulation and transformation be substantially underrated with respect to the normal treater loads.
The full wave and half wave rectifying system employed in conventional power supplies has basically the same problem as in the aforementioned patents. Particularly, an unbalance in current consumption at the output terminals causes an unbalanced a.c. component to be passed through one of the rectifiers. This a.c. component appears as a d.c. current in the secondary winding to magnetize the core of the transformer and results in greater losses within the transformer, and leads to saturation of conventional current limiting reactors. The electrical field treater of the present invention provides a novel power supply which is capable of producing simultaneous energization of an insulated set of electrodes with both a.c. and d.c. voltage components wherein the d.c. components are regulated at a first potential between the electrodes and at a second potential relative to earth grounds in the treater. In addition, either electrode can consume more current than the other, creating a substantially severe current unbalance and not magnetizing the transformer employed in the power source. In addition, the magnitude of the a.c. component present between the sets of electrodes and earth grounds is arranged to a selected value.
It is usual to employ current limiting reactors in the primary circuits of electric treater power sources. The transformers normally employed for full wave rectification must be derated to approximately 35% output current rating when supplying loads at half wave rectified d.c. The danger is that the regulator means will be set to protect the transformer assuming balanced secondary operation, thus leaping to excessive (.times.3) overload operation should one of the half wave d.c. load fields collapse in the treater due to insulation failure or increases in localized demand.